US7170961B2  Method and apparatus for frequencydomain tracking of residual frequency and channel estimation offsets  Google Patents
Method and apparatus for frequencydomain tracking of residual frequency and channel estimation offsets Download PDFInfo
 Publication number
 US7170961B2 US7170961B2 US10042780 US4278002A US7170961B2 US 7170961 B2 US7170961 B2 US 7170961B2 US 10042780 US10042780 US 10042780 US 4278002 A US4278002 A US 4278002A US 7170961 B2 US7170961 B2 US 7170961B2
 Authority
 US
 Grant status
 Grant
 Patent type
 Prior art keywords
 carrier
 signal
 phase
 frequency
 offset
 Prior art date
 Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
 Active, expires
Links
Images
Classifications

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/26—Systems using multifrequency codes
 H04L27/2601—Multicarrier modulation systems
 H04L27/2647—Arrangements specific to the receiver
 H04L27/2655—Synchronisation arrangements
 H04L27/2668—Details of algorithms
 H04L27/2673—Details of algorithms characterised by synchronisation parameters

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/26—Systems using multifrequency codes
 H04L27/2601—Multicarrier modulation systems
 H04L27/2647—Arrangements specific to the receiver
 H04L27/2655—Synchronisation arrangements
 H04L27/2657—Carrier synchronisation
 H04L27/2659—Coarse or integer frequency offset determination and synchronisation

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/26—Systems using multifrequency codes
 H04L27/2601—Multicarrier modulation systems
 H04L27/2647—Arrangements specific to the receiver
 H04L27/2655—Synchronisation arrangements
 H04L27/2657—Carrier synchronisation
 H04L27/266—Fine or fractional frequency offset determination and synchronisation

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/26—Systems using multifrequency codes
 H04L27/2601—Multicarrier modulation systems
 H04L27/2647—Arrangements specific to the receiver
 H04L27/2655—Synchronisation arrangements
 H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
 H04L27/2695—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation with channel estimation, e.g. determination of delay spread, derivative or peak tracking
Abstract
Description
This application relates to commonly assigned copending U.S. patent application Ser. No. 09/966,419, filed Sep. 27, 2001, titled “Method and Apparatus for Channel Estimation,” by Patrick VandenameeleLepla, the disclosure of which is hereby incorporated by reference in its entirety for all purposes.
Not Applicable
Not Applicable
The present invention relates in general to communication systems, and in particular to methods and systems for improving various aspects of communication systems utilizing multicarrier transmission techniques such as orthogonal frequency division multiplexing.
Wireless personal communication devices have proliferated over the past several years. Integration of more functionality such as multimedia capabilities into these devices has created an ever increasing demand for enhanced broadband communication methodologies. Unlike satellite communication where there is a single direct path from a transmitter to a receiver, personal wireless communication devices must operate in a multipath environment. Multipath propagation is caused by the transmitted signal reflecting off of objects such as buildings, automobiles, trees, etc., that may be encountered along the signal path. This results in the receiver receiving multiple copies of the transmitted signal each having different delay, attenuation and phase shift depending on the length of the path and the material composition of the objects along the path. The interference between the multiple versions of the transmitted signal, referred to as intersymbol interference (ISI), is a common problem that can severely distort the received signal.
Orthogonal frequency division multiplexing (OFDM) is one type of multicarrier data transmission technique that has had some success in addressing ISI, distortion and other problems associated with multipath environments. OFDM divides the available spectrum into multiple carriers, each one being modulated by a low rate data stream. Multiple user access is achieved by subdividing the available bandwidth into multiple channels, that are then allocated to users. The orthogonality of the carriers refers to the fact that each carrier has an integer number of cycles over a symbol period. Due to this, the spectrum of each carrier has a zero at the center frequency of each of the other carriers in the system. This results in no interference between the carriers, allowing them to be spaced as close as theoretically possible. Each carrier in an OFDM signal has a very narrow bandwidth, thus the resulting symbol rate is low. This results in the signal having a high tolerance to multipath delay spread, as the delay spread must be very long to cause significant intersymbol interference. Coded orthogonal frequency division multiplexing (COFDM) is the same as OFDM except that forward error correction is applied to the signal before transmission. This is to overcome errors in the transmission due to lost carriers from frequency selective fading, channel noise and other propagation effects. In the description presented herein, the terms OFDM and COFDM are used interchangeably.
In OFDM the subcarrier pulse used for transmission is chosen to be rectangular. This allows the task of pulse forming and modulation to be performed by an inverse discrete Fourier transform (IDFT). IDFT is implemented very efficiently as an inverse fast Fourier transform (IFFT) which would then require only an FFT at the receiver end to reverse the process. In order to accurately reproduce the transmitted signal, the receiver must estimate and compensate for various distortions in the received signal, including channel transfer function, carrier frequency offset and clock frequency offset.
The channel transfer function reflects the effect of the propagation channel on a transmitted signal, including multipath delay effects. A number of techniques are known for estimating and compensating for the channel. Channel estimation may be performed in the receiver either in the time domain (i.e., before the receiver performs the FFT on the received signal) or in the frequency domain (i.e., after the receiver performs the FFT on the received signal). Compensation for the channel using the channel estimate is generally performed in the frequency domain.
Carrier frequency offset (CFO) arises from the finite tolerance of RF components used in the transmitter and receiver. Even though the transmitter frequency is usually known to the receiver, due to RF tolerance, the frequency at which the receiver operates may not match exactly. This offset causes a phase shift in the received signal. Additionally, in systems where the transmitter or receiver is mobile, Doppler effects may also give rise to carrier frequency offsets. In order to provide accurate reproduction of the transmitted signal, this CFOinduced phase shift must be estimated and compensated by the receiver.
Clock frequency offset (CLO) arises from a mismatch between the sampling rate at the receiver and the sampling rate at the transmitter; again, the mismatch causes a phase shift in the received signal that must be estimated and compensated by the receiver. In addition, CLO may cause synchronization errors if the receiver assigns too many or too few samples to a symbol. Some OFDM standards, such as the 802.11a standard, reduce the complexity of the clock and carrier frequency offset problem by requiring that the carrier frequency and clock frequency in the receiver be derived from the same oscillator. But these standards do not eliminate the problem since the frequency of the receiver's oscillator will in general not exactly match the frequency of a transmitter's oscillator.
A number of methods for estimating phase shifts caused by CFO and/or CLO have been proposed. Some of these methods operate on received signals in the time domain (i.e., before the FFT is performed by the receiver); these include computations based on time correlations. Others operate in the frequency domain (i.e., after the FFT is performed by the receiver) and may involve least squares or maximum likelihood computations.
Existing methods for estimating phase shifts assume that the only noise present is additive white Gaussian noise, i.e., that the noise power spectrum is independent of carrier frequency. In many implementations of OFDM, however, nonwhite noise (i.e., noise with a power spectrum that depends on frequency, such as 1/f noise) may be present due to the receiver front end electronics or other noise sources. This nonwhite noise adversely affects the existing phase estimation methods, leading to increased error rates. Thus, a need exists for a method of computing carrier frequency and clock frequency offsets that does not rely on the assumption of white noise.
The present invention provides methods and systems for efficiently implementing broadband multicarrier communication systems. In one embodiment, the invention provides a frequencydomain estimate of carrier frequency offsets that is robust against nonwhite (frequencydependent) noise. Specifically, a carrierspecific weighting factor is associated with each of the carriers of a multicarrier signal. A set of received signals and estimated channel transfer functions associated with each of the carriers are then used together with the carrierspecific weighting factors to compute a carrier frequency offset. This carrier frequency offset estimate may then be applied in a feedback loop to phasecompensate a subsequently received signal. Because greater weights are given to carriers whose frequencies include lower noise and/or carriers transmitting a known sequence of symbols, the estimate of the carrier frequency offset is improved, resulting in more reliable reproduction of a transmitted multicarrier signal by a receiver. Additionally, in embodiments where the multicarrier communication system conforms to certain standards, the carrier frequency offset estimate may be used to compute a clock frequency offset estimate. The clock frequency offset estimate may then be used to accomplish farther improvement of signal synchronization. These and other advantages flow from the methods and systems of the present invention.
According to one aspect of the present invention, in a multicarrier data communication system, a method of estimating a carrier frequency offset for a received signal is provided. The method comprises associating each of a plurality of carrierspecific weighting factors with a different one of a plurality of carriers of the multicarrier data and assigning a value to each of the plurality of carrierspecific weighting factors, the value being related to a noise power associated with the associated carrier. A carrier frequency offset estimate is then computed using the received signal, an estimate of a channel transfer function associated with the received signal, and the plurality of carrierspecific weighting factors. This carrier frequency offset estimate may then be used to phase compensate a subsequent received signal.
The values of the carrierspecific weighting factors may be assigned by measuring a noise power spectrum across the plurality of carriers, then selecting a value inversely proportional to the noise power for one of the plurality of carriers and assigning the selected value to the associated carrierspecific weighting factor. Further, in embodiments where the plurality of carriers includes a first subset of pilot carriers, which carry a known sequence of symbols, and a second subset of nonpilot carriers, a first carrierspecific weighting factor associated with one of the pilot carriers may be increased relative to a second carrierspecific weighting factor associated with one of the nonpilot carriers. Still further, in embodiments where a first carrier has an associated channel estimate that has higher reliability than a channel estimate associated with a second carrier, a first carrierspecific weighting factor associated with the first carrier may be increased relative to a second carrierspecific weighting factor associated with the second carrier.
The carrier frequency offset estimate may be computed by phasecompensating the received signal using a previous carrier frequency offset estimate; equalizing the phasecompensated signal using the estimate of the channel transfer function; computing a phase metric from the phasecompensated signal, the equalized signal, the estimate of the channel transfer function, and the plurality of carrierspecific weighting factors; computing a phase of the phase metric; and applying a loop filter to the computed phase. The phase metric may be computed by applying a threshold cutoff to the equalized signal, thereby producing a sliced signal; multiplying the phasecompensated signal for each of the plurality of carriers by the complex conjugate of the sliced signal for the carrier and by the complex conjugate of the channel estimate for the carrier, thereby obtaining a first product; multiplying the first product by the carrierspecific weight associated with the carrier, thereby obtaining a weighted product; and summing the weighted product over the plurality of carriers, thereby obtaining the phase metric.
According to another aspect of the invention, a method for processing a multicarrier signal transmitted across a channel is provided. A value is assigned to each of a plurality of carrierspecific weighting factors, each of the plurality of carrierspecific weighting factors being associated with a different one of a plurality of carriers of the multicarrier signal, and the assigned value of each carrierspecific weighting factor being related to a noise power associated with the carrier. The multicarrier signal is received and phasecompensated using a phase compensation factor, then equalized using a channel estimate. A carrier frequency offset is estimated using the phasecompensated signal, the equalized signal, the channel estimate, and the plurality of carrierspecific weighting factors. A clock frequency offset is estimated using the estimated carrier frequency offset. The phase compensation factor is updated using the estimated carrier frequency offset and the estimated clock frequency offset.
The carrier frequency offset may be estimated by computing a phase metric from the phasecompensated signal, the equalized signal, the channel estimate, and the plurality of carrierspecific weighting factors; computing a phase of the phase metric; and applying a loop filter to the computed phase. The loop filter may comprise an infinite impulse response filter. The phase metric may be computed by applying a threshold cutoff to the equalized signal, thereby producing a sliced signal; multiplying the phasecompensated signal for each of the plurality of carriers by the complex conjugate of the sliced signal for the carrier and by the complex conjugate of the channel estimate for the carrier, thereby obtaining a first product; multiplying the first product by the carrierspecific weight associated with the carrier, thereby obtaining a weighted product; and summing the weighted product over the plurality of carriers, thereby obtaining the phase metric. The clock frequency offset may be computed by multiplying the estimated carrier frequency offset by a factor inversely proportional to a carrier frequency.
According to yet another aspect of the invention, the clock frequency offset estimate may be used to provide improved synchronization of the received signal. A net time offset may be computed based on the clock offset estimate and an elapsed time. A drop instruction may be generated when the net time offset exceeds a drop threshold, the drop instruction causing a portion of the multicarrier signal to be dropped from a symbol, and an add instruction may be generated when the net time offset is below an add threshold, the add instruction causing a portion of the multicarrier signal to be added to the symbol. The elapsed time may be reset after the portion of the multicarrier signal has been added to or dropped from the symbol.
According to a further aspect of the invention, a coarse carrier frequency offset and a fine carrier frequency offset may be determined. The phase compensation factor may be updated using the coarse carrier frequency offset and the fine carrier frequency offset.
According to a still further aspect of the invention, in a multicarrier data communication system, a method of equalizing a multicarrier signal is provided. The method includes estimating a channel transfer function; compensating a received signal using a phase compensation factor, yielding a phasecompensated signal; compensating the phasecompensated signal using the estimated channel transfer function, yielding an equalized signal; estimating a phase metric using the phasecompensated signal the equalized signal, the estimated channel transfer function, and a plurality of carrierspecific weighting factors, wherein each of the carrierspecific weighting factors being associated with a different one of a plurality of carriers of the multicarrier signal and assigned a value related to a noise power associated with the carrier; estimating a carrier frequency offset using the estimated phase metric; estimating a clock frequency offset using the updated estimate of the carrier frequency offset; and updating the phase compensation factor using the estimated carrier frequency offset and the estimated clock frequency offset. The value of each of the carrierspecific weighting factors may be inversely proportional to a noise power associated with the associated carrier. Where the plurality of carriers comprises a first subset of pilot carriers and a second subset of nonpilot carriers, the carrierspecific weighting factor associated with at least one of the pilot carriers may be increased relative to the carrierspecific weighting factor associated with at least one of the nonpilot carriers.
A phase metric may be estimated by applying a threshold cutoff to the equalized signal, thereby producing a sliced signal; multiplying the phasecompensated signal for each carrier by the complex conjugate of the sliced signal for the carrier and by the complex conjugate of the estimated channel transfer function for the carrier, thereby obtaining a product; multiplying the product by the carrierspecific weight associated with the carrier, thereby obtaining a weighted product; and summing the weighted product over the plurality of carriers, thereby obtaining the phase metric.
The carrier frequency offset may be estimated by computing a phase metric from the phasecompensated signal, the equalized signal, the channel estimate, and the plurality of carrierspecific weighting factors; computing a phase of the phase metric; and applying a loop filter to the computed phase. The clock offset may be estimated by multiplying the estimated carrier frequency offset by a factor inversely proportional to the carrier frequency.
According to another aspect of the invention, in a receiver for a multicarrier data communication system, an equalizer is provided. The equalizer comprises a phase compensator configured to receive the input sample, a carrier frequency phase offset estimate, and a clock frequency phase offset estimate, and to output a phase compensated sample; a channel equalization block configured to receive a plurality of channel estimates and the phase compensated sample, and to output an equalized data sample; a carrier frequency offset estimator configured to receive the plurality of channel estimates, the phase compensated sample, and the equalized sample, and to compute and output the carrier frequency phase offset estimate using a plurality of carrierspecific weighting factors, each of the carrierspecific weighting factors being associated with a different one of a plurality of carriers of the multicarrier data and having a value related to a noise power associated with the associated carrier; and a clock frequency offset estimator configured to receive the carrier frequency phase offset estimate and compute the clock frequency phase offset estimate.
The carrier frequency offset estimator may comprise a weight source configured to output the plurality of carrierspecific weighting factors; a slicer configured to receive the equalized signal and to output a sliced signal; a phase metric updater configured to receive the plurality of channel estimates, the phasecompensated signal, the sliced signal, and the plurality of carrierdependent weights, and to compute and output a phase metric; a phase computation unit coupled to the phase metric updater and configured to compute and output a phase of the phase metric; and a loop filter coupled to the phase computation unit and configured to store a plurality of values of the phase and to compute the carrier frequency phase offset estimate. The weight source may comprise a noise estimator configured to measure a noise power spectrum.
According to yet another aspect of the invention, a multicarrier data communication system is provided. The system comprises a transmitter and a receiver. The transmitter includes a demodulator/deserializer configured to convert an input data stream into a parallel plurality of multicarrier signals; a frequencydomain to timedomain converter having an input coupled to the modulator/deserializer and configured to transform the parallel plurality of multicarrier signals from frequency domain into time domain at an output; a guard period insertion block coupled to the frequencydomain to timedomain converter and configured to insert a guard period in the output of the frequencydomain to timedomain converter; a serializer coupled to an output of the guard period insertion block and configured to perform a parallel to serial conversion on the signal; and a digitaltoanalog converter coupled to the serializer and configured to convert the digital signal into an analog signal and to transmit the analog multicarrier time domain signal across a channel. The receiver includes an analogtodigital converter coupled to receive the analog signal and configured to convert the analog signal into a digital signal; a deserializer coupled to the analogtodigital converter and configured to convert the digital signal into a plurality of parallel signals; a channel estimator coupled to an output of the deserializer and configured to compute a channel transfer function estimate; a guard period removal block coupled to an output of the channel estimator and configured to remove the guard period; a timedomain to frequencydomain converter coupled to an output of the guard period removal block; an equalizer coupled to an output of the timedomain to frequencydomain converter, configured to equalize the signal using the channel estimates and further configured to compensate for a carrier frequency offset and a clock offset using a carrier frequency offset estimate that includes a plurality of carrierspecific weighting factors, each of the carrierspecific weighting factors being associated with a different one of a plurality of carriers of the multicarrier data and having a value related to a noise power associated with the associated carrier; and a serializer/demodulator coupled to the output of the equalizer and configured to generate an output data stream.
The multicarrier data communication system may also include a preliminary carrier frequency offset estimation block coupled between the deserializer and the guard period removal block, the preliminary carrier frequency offset estimation block configured to output a preliminary estimate of carrier frequency offset. The equalizer may be configured to receive the preliminary estimate of carrier frequency offset for use in compensating for the carrier frequency offset.
The following detailed description along with the accompanying drawings will provide a better understanding of the nature and advantages of the present invention.
In one embodiment, the present invention provides an estimate of carrier frequency offset and clock frequency offset that is robust against the presence of nonwhite noise. Referring to
At the receiver end, an analogtodigital (A/D) converter 116 receives the analog signal and converts it into a digital signal. The digital timedomain signal is first deserialized by a deserializer 118. Optionally, a preliminary estimation of and compensation for carrier frequency offset (CFO) is performed by a preliminary CFO estimator 120. A number of techniques for estimating carrier frequency offset in the time domain are known in the art and may be applied in preliminary CFO estimator 120. When preliminary CFO estimator 120 is used, a preliminary CFO estimate may be provided to downstream components.
The signal is then fed into a channel estimator 122, which computes channel estimates, for instance by processing a training sequence that is embedded in the data stream at the transmitter end. One embodiment of a channel estimator 122 that can be advantageously employed in communication system 100 is described in detail in the abovecrossreferenced copending U.S. patent application Ser. No. 09/966,419, titled “Method and Apparatus for Channel Estimation.” Other channel estimators may also be used. The channel estimates are provided as an output for later use by downstream components, either by adding the channel estimates back into the data stream or by providing a separate data path (not shown). In either case, the data stream from channel estimator 122 is applied to a block 124 which determines the beginning of each symbol and removes the guard period from the timedomain signal. With the guard period removed, an FFT block 126 transforms the timedomain signal into a frequency domain signal.
An equalizer 128 receives the output of FFT block 126 and generates an equalized output signal. Equalizer 128 includes a phase estimator 130 and a phase compensator 132. Phase estimator 130 produces estimates of the carrier frequency offset and clock frequency offset, taking into account frequencydependent (nonwhite) noise in the system, as will be described further below. Phase compensator 132 uses phase estimates provided by phase estimator 130 to compensate the received signal. Equalizer 128 also performs channel equalization on the signal using the channel estimates received from channel estimator 122. The equalized signal is then serialized by a serializer 134 and demodulated by a block 136 to generate an output data stream.
It is to be understood that the data communication system as depicted in
Equalizer 128 provides phase compensation via a feedback loop. The received signal R is input to phase compensator 132, which adjusts the phase of received signal R by multiplying it by exp(j2πφ), where φ is an estimate of the total phase offset, including both carrier and clock frequency offsets. Phase compensator 132 produces a phasecompensated signal R′. Phasecompensated signals R′ are provided to phase estimator 130, which uses the signals R′ together with other data to generate a carrier frequency offset (CFO) estimate (dF) and a clock frequency offset (CLO) estimate (dClk). These estimates are in turn used by phase compensator 132 to compute total phase offset φ for compensating subsequent signals.
Phasecompensated signal R′ is also provided to a division block 205, which performs compensation for the channel transfer function by dividing the phasecompensated signal R′ by channel estimates (H) provided by a channel estimate source 210. Channel estimate source 210 may comprise a memory for storing channel estimates received from channel estimator 122 of
Phase estimator 130 receives phasecompensated signal R′, equalized signal X, and channel estimates H as inputs, and provides phase offset estimates dF and dClk to compensate phase block 132. Phase estimator 130 also receives a sliced signal ({circumflex over (X)}), which is derived from equalized signal X by slicer 215, which applies a hard threshold cutoff. Phase estimator 130 comprises CFO estimator 220 and CLO estimator 225, which will now be described in more detail.
For each carrier k, phase metric updater 305 computes a carrier phase metric component (C_{k}). In general, the phase difference between a phasecompensated sample R_{k}′ and a corresponding sliced sample {circumflex over (X)}_{k }may be obtained by multiplying R_{k}′ by the complex conjugate of {circumflex over (X)}_{k }(denoted {circumflex over (X)}_{k}*). An improved estimate of the phase offset may be obtained if phasecompensated sample R_{k}′ is first multiplied by the complex conjugate of the corresponding channel estimate H_{k }(denoted H_{k}*). This reduces any phase change introduced by the channel while giving greater weight to carriers in which the channel transfer function is large, i.e., where the signal is strongest. Thus, carrier phase metric component C_{k }is given by:
C _{k} =H _{k} *{circumflex over (X)} _{k} *R _{k}′.
Carrier phase metric components C_{k }are computed for each carrier k.
A phase metric (PM) is then computed by forming a weighted sum of carrier phase metric components C_{k}, where each C_{k }is weighted by the corresponding carrier dependent weight w_{k}:
Phase metric updater 305 provides phase metric PM as a complex number to arctan block 315. Arctan block 315 computes and outputs the phase of phase metric PM, using the wellknown property that for any complex number Z, the phase of Z is given by arctan(Im Z/Re Z), where Im Z is the imaginary part of Z and Re Z is the real part of Z.
Arctan block 315 is followed by a loop filter 320, which combines current and previously computed phase values to determine the CFO estimate dF. In one embodiment, loop filter 320 is advantageously implemented as an infinite impulse response filter, in which all previously received phase values are used, but the weight given to each value decreases with the time since receipt. Such filters are well known in the art, and it will be appreciated that an infinite impulse response filter provides stability in the feedback loop without unnecessarily compromising the responsiveness of the CFO estimate dF. However, other loop filters, such as time averaging over a fixed number of symbol periods, may also be employed.
Exemplary techniques for selecting values for the carrierdependent weights w_{k }will now be described. As stated above, for each carrier k, weight source 310 provides a carrierdependent weight w_{k }to be used in computing phase metric PM. The values of the carrierdependent weights are controlled to account for the effect of frequencydependent noise in the system, so that those carrier frequencies that are subject to greater noise are given relatively less weight in determining phase metric PM, while carrier frequencies subject to less noise are given relatively greater weight. Preferably, a noise power spectrum is estimated for the carriers k, and the values of weights w_{k }are selected to be inversely proportional to the noise power spectrum. Other relationships between noise power and weight may also be used. Although the carrierdependent weights w_{k }may be selected to be normalized (e.g., by requiring Σw_{k}=1), it will be appreciated that this is not necessary because only the phase, not the amplitude, of phase metric PM is of interest.
Numerous techniques for estimating a noise power spectrum are known in the art, and any such technique may be employed in weight source 310. For instance, in embodiments where the frequencydependent noise is dominated by noise within the receiver electronics, the noise power spectrum may be measured offchip and values for carrierdependent weights w_{k }programmed into weight source 310.
In other embodiments, carrierdependent weights w_{k }may be measured in real time from the data itself. For example,
Noise estimator 410 may also be configured to generate weights during a calibration event when no data is being transmitted over channel 114 of
Accordingly, noise estimator 410 may be configured to receive the calibration signal Calib as shown in
In addition, in some implementations of OFDM, a subset of the carriers transmit a known sequence of symbols to be used by the receiver for computing channel estimates and for other purposes. These “pilot” carriers provide reliable estimates of phase offset because the transmitted signal in each pilot carrier is known. Thus, weight source 310 or noise estimator 410 may be configured to increase the carrierdependent weights w_{k }for the pilot carriers relative to other carriers. This feature may be implemented in combination with any of the above or other methods of estimating a noise power spectrum.
As another example, in some implementations of OFDM, some of the carriers provide more reliable channel estimates than others, and the reliability of the channel estimate for each carrier is known in advance. For example, the channel estimation system and method disclosed in the abovecrossreferenced U.S. patent application Ser. No. 09/966,419 provides a channel estimate that has a known reliability distribution for the carriers. Reliability may be measured, e.g., by relative uncertainty in the channel estimate, by absolute uncertainty in the channel estimate, or by other standard measures. In such implementations, weight source 310 or noise estimator 410 may be configured to increase or decrease the carrierdependent weight w_{k }as a function of the known reliability of the channel estimate for the carrier, thereby giving greater relative weight to carriers with more reliable channel estimates. This feature may also be implemented in combination with any of the above or other methods of estimating a noise power spectrum.
It will be appreciated that the foregoing examples of a weight source do not exhaust the possibilities for estimating a noise power spectrum and computing weights. Indeed, any noise estimation technique that provides an estimate of a noise power spectrum may be used.
Returning to
CLO estimate dClk represents the difference between the sampling clock rates in the transmitter and the receiver. As described above, this difference may lead to errors if the receiver incorrectly identifies the beginning of a symbol. In some embodiments, CLO estimator 225 may be used to provide improved synchronization, as will now be described.
A signal Adropped is sent to equalizer 128 by guard period removal block 124 when it has added or dropped a signal either in response to a received add or drop command or on the basis of its internal synchronization algorithms (a number of which are known in the art). The signal Adropped is received by phase compensator 132, which then shifts the phase by one sample; the shift may be either up or down, depending on whether signal Adropped indicates an add or a drop. Signal Adropped is also provided to Add/Drop module 550, causing a reset of the elapsed time.
In many implementations, it is advantageous to perform a preliminary estimation of and compensation for CFO in the time domain, for instance via preliminary CFO block 120 of
In such an implementation, there may remain a residual CFO that is not removed by the timedomain estimation. For instance, a typical timedomain estimation algorithm cannot adequately deal with phase offsets in the presence of nonwhite noise (i.e., a noise power spectrum that depends on carrier frequency). In addition, preliminary CFO block 120 generally does not estimate or compensate for clock offset.
The phase offset estimation procedures described above are equally useful in such implementations. For example,
FCFO is provided as an input to phase compensator 632 and to CLO estimator 625. CCFO is provided as an input to CLO estimator 625. CLO estimator 625 is similar to previously described CLO estimator 225, except that the CFO estimate used is the sum of CCFO, FCFO and a residual CFO estimate dFres provided by residual CFO estimator 620. The operation of residual CFO estimator 620 is essentially the same as described above with regard to
The preliminary carrier frequency offset estimates CCFO and FCFO may also be used during system initialization. For example, in the embodiment of
The present invention thus provides various methods and systems that efficiently implement broadband multicarrier communication systems. Estimation of carrier frequency offset according to the present invention results in reduced error in the presence of nonwhite noise. In addition, the present invention provides improved synchronization by using an estimate of clock frequency offset derived from the carrier frequency offset estimate.
While a detailed description of exemplary embodiments of the invention has been presented, it will be appreciated that various alternatives, modifications, and equivalents are possible. For instance, weight source 310 may be implemented using any noise estimation technique, and its inputs and outputs may be varied accordingly. It is further to be understood that any of the functional blocks in equalizer 128 may be implemented by a combination of hardware and/or software, and that in specific implementations some or all of the functionality of some of the blocks may be combined. Also, while the techniques described herein are particularly well suited for wireless communication systems using OFDM, similar advantages can be realized when applying the same to wireline systems, and in general any system requiring compensation for phase errors induced by carrier frequency offsets. The scope of the present invention is thus not limited to the specific embodiments described, and is instead defined by the following claims and their fall breadth of equivalents.
Claims (29)
Priority Applications (1)
Application Number  Priority Date  Filing Date  Title 

US10042780 US7170961B2 (en)  20020108  20020108  Method and apparatus for frequencydomain tracking of residual frequency and channel estimation offsets 
Applications Claiming Priority (1)
Application Number  Priority Date  Filing Date  Title 

US10042780 US7170961B2 (en)  20020108  20020108  Method and apparatus for frequencydomain tracking of residual frequency and channel estimation offsets 
Publications (2)
Publication Number  Publication Date 

US20030128751A1 true US20030128751A1 (en)  20030710 
US7170961B2 true US7170961B2 (en)  20070130 
Family
ID=21923714
Family Applications (1)
Application Number  Title  Priority Date  Filing Date 

US10042780 Active 20240210 US7170961B2 (en)  20020108  20020108  Method and apparatus for frequencydomain tracking of residual frequency and channel estimation offsets 
Country Status (1)
Country  Link 

US (1)  US7170961B2 (en) 
Cited By (11)
Publication number  Priority date  Publication date  Assignee  Title 

US20040202234A1 (en) *  20030411  20041014  Agency For Science, Technology And Research  Lowcomplexity and fast frequency offset estimation for OFDM signals 
US20050152326A1 (en) *  20040108  20050714  Rajiv Vijayan  Frequency error estimation and frame synchronization in an OFDM system 
US20060291549A1 (en) *  20050627  20061228  Nokia Corporation  Automatic receiver calibration with noise and fast fourier transform 
US20070183537A1 (en) *  20060208  20070809  Nec Corporation  Radio receiver and noise estimated value correction method 
US20080205540A1 (en) *  20040528  20080828  Daisuke Takeda  Wireless communication apparatus 
WO2008115782A1 (en) *  20070321  20080925  Nextwave Broadband Inc.  Methods and apparatus for identifying subscriber station mobility 
US20100150283A1 (en) *  20070404  20100617  Thomson Licensing A Corporation  Method and apparatus for digital signal reception 
US20110149724A1 (en) *  20091217  20110623  Electronics And Telecommunications Research Institute  Apparatus and method for transmitting/receiving data in wireless communication system 
CN102185811A (en) *  20110524  20110914  中国工程物理研究院电子工程研究所  Carrier frequency estimation method 
US8311152B1 (en) *  20040227  20121113  Marvell International Ltd.  Adaptive OFDM receiver based on carrier frequency offset 
US20130170590A1 (en) *  20100917  20130704  Telefonaktiebolaget L M Ericsson (Publ)  Receiver Node and A Method Therein for Compensating Frequency Offset 
Families Citing this family (21)
Publication number  Priority date  Publication date  Assignee  Title 

US7266162B2 (en) *  20020618  20070904  Lucent Technologies Inc.  Carrier frequency offset estimator for OFDM systems 
US7272108B2 (en) *  20020801  20070918  Mediatek, Inc.  Channel estimation in orthogonal frequencydivision multiplexing (OFDM) systems 
US8457230B2 (en) *  20020821  20130604  Broadcom Corporation  Reconfigurable orthogonal frequency division multiplexing (OFDM) chip supporting single weight diversity 
KR100542827B1 (en) *  20021218  20060120  한국전자통신연구원  Deltavaluepredicted frequency offset compensation apparatus and method thereof 
DE10334842B4 (en) *  20030730  20050602  Infineon Technologies Ag  Weighting circuit for a multicarrier signal receiver 
US7362802B2 (en) *  20030912  20080422  Zarbana Digital Fund Llc  Frequency domain equalizer for wireless commuications system 
US20050059366A1 (en) *  20030916  20050317  Atheros Communications, Inc.  Spur mitigation techniques 
US7417945B2 (en) *  20031002  20080826  Texas Instruments Incorporated  Transmitter and receiver for use with an orthogonal frequency division multiplexing system 
US7529179B1 (en) *  20050211  20090505  Marvell International Ltd.  Joint maximum likelihood estimation of integer carrier frequency offset and channel in OFDM systems 
US7177374B2 (en) *  20050617  20070213  Broadcom Corporation  Apparatus and method for sampling frequency offset estimation and correction in a wireless communication system 
US7526020B2 (en) *  20050913  20090428  Via Technologies, Inc.  Circuit for improving channel impulse response estimation and compensating for remnant frequency offset in the orthogonal frequency division multiplexing (OFDM) baseband receiver for IEEE 802.11a/g wireless LAN standard 
US20070070934A1 (en) *  20050928  20070329  Pieter Van Rooyen  Method and system for a reconfigurable OFDM radio supporting diversity 
KR20070039249A (en) *  20051007  20070411  삼성전자주식회사  Apparatus and method for twodimensional equalization in a communication system using an orthogonal frequency division multiple access scheme 
US7872962B1 (en) *  20051018  20110118  Marvell International Ltd.  System and method for producing weighted signals in a diversity communication system 
KR100772504B1 (en) *  20051201  20071101  한국전자통신연구원  OFDM intensity modulation and direct detection apparatus and method with high transmission power efficiency in wire/wireless communication system 
US7889813B2 (en) *  20051214  20110215  Oki Techno Center (Singapore) Pte Ltd.  Method, apparatus and receiver for demapping dual carrier modulated COFDM signals 
US7852971B2 (en) *  20060721  20101214  Qualcomm, Incorporated  False channel detection for wireless communication 
KR100910715B1 (en) *  20071204  20090804  한국전자통신연구원  Multistage channel estimation method and apparatus 
US8170160B1 (en) *  20081119  20120501  Qualcomm Atheros, Inc.  Multisymbol phase offset estimation 
US9461855B2 (en)  20120705  20161004  Intel Corporation  Methods and arrangements for selecting channel updates in wireless networks 
US9413380B2 (en) *  20140314  20160809  Stmicroelectronics S.R.L.  High performance digital to analog converter 
Citations (11)
Publication number  Priority date  Publication date  Assignee  Title 

US5228062A (en)  19900416  19930713  Telebit Corporation  Method and apparatus for correcting for clock and carrier frequency offset, and phase jitter in multicarrier modems 
US5282222A (en)  19920331  19940125  Michel Fattouche  Method and apparatus for multiple access between transceivers in wireless communications using OFDM spread spectrum 
US5450456A (en)  19931112  19950912  Daimler Benz Ag  Method and arrangement for measuring the carrier frequency deviation in a multichannel transmission system 
US5487069A (en)  19921127  19960123  Commonwealth Scientific And Industrial Research Organization  Wireless LAN 
EP1172956A1 (en) *  19990422  20020116  Nippon Telegraph and Telephone Corporation  OFDM packet communication receiver 
US6442211B1 (en) *  19960920  20020827  IAD Gesellschaft für Informatik, Automatisierung und Datenverarbeitung mbH  System for digital information transmission with associated methods and devices 
US20030086504A1 (en) *  20011105  20030508  Magee David Patrick  System and method for soft slicing 
US20030112902A1 (en) *  20011213  20030619  Koninklijke Philips Electronics N.V.  Bit level diversity combining for COFDM system 
US6628738B1 (en) *  19970922  20030930  Alcatel  Method of arrangement to determine a clock timing error in a multicarrier transmission system, and a related synchronization units 
US6704374B1 (en) *  20000216  20040309  Thomson Licensing S.A.  Local oscillator frequency correction in an orthogonal frequency division multiplexing system 
US6862262B1 (en) *  19990913  20050301  Matsushita Electric Industrial Co., Ltd.  OFDM communication device and detecting method 
Family Cites Families (3)
Publication number  Priority date  Publication date  Assignee  Title 

US5682376A (en) *  19941220  19971028  Matsushita Electric Industrial Co., Ltd.  Method of transmitting orthogonal frequency division multiplex signal, and transmitter and receiver employed therefor 
JP2002009734A (en) *  20000627  20020111  Denso Corp  Communication system employing ofdm system 
US6928120B1 (en) *  20000925  20050809  Cingular Wireless Ii, Llc  Methods and apparatus for use in reducing residual phase error in OFDM communication signals 
Patent Citations (11)
Publication number  Priority date  Publication date  Assignee  Title 

US5228062A (en)  19900416  19930713  Telebit Corporation  Method and apparatus for correcting for clock and carrier frequency offset, and phase jitter in multicarrier modems 
US5282222A (en)  19920331  19940125  Michel Fattouche  Method and apparatus for multiple access between transceivers in wireless communications using OFDM spread spectrum 
US5487069A (en)  19921127  19960123  Commonwealth Scientific And Industrial Research Organization  Wireless LAN 
US5450456A (en)  19931112  19950912  Daimler Benz Ag  Method and arrangement for measuring the carrier frequency deviation in a multichannel transmission system 
US6442211B1 (en) *  19960920  20020827  IAD Gesellschaft für Informatik, Automatisierung und Datenverarbeitung mbH  System for digital information transmission with associated methods and devices 
US6628738B1 (en) *  19970922  20030930  Alcatel  Method of arrangement to determine a clock timing error in a multicarrier transmission system, and a related synchronization units 
EP1172956A1 (en) *  19990422  20020116  Nippon Telegraph and Telephone Corporation  OFDM packet communication receiver 
US6862262B1 (en) *  19990913  20050301  Matsushita Electric Industrial Co., Ltd.  OFDM communication device and detecting method 
US6704374B1 (en) *  20000216  20040309  Thomson Licensing S.A.  Local oscillator frequency correction in an orthogonal frequency division multiplexing system 
US20030086504A1 (en) *  20011105  20030508  Magee David Patrick  System and method for soft slicing 
US20030112902A1 (en) *  20011213  20030619  Koninklijke Philips Electronics N.V.  Bit level diversity combining for COFDM system 
NonPatent Citations (20)
Title 

"dspGuru: Invinite Impulse Response Filter FAQ", Loweigan International, 19992004, pags 12. * 
Beek et al., "ML Estimation of Time and Frequency Offset in OFDM Systems," IEEE Trans. on Signal Processing, vol. 45, No. 7, pp. 18001805, (Jul. 1997). 
Boelcskei, "Blind estimation of symbol Timing and Carrier Frequency Offset in Pulse Shaping OFDM Systems," Proc. of IEEE Int. Conf. on Acoustics, Speech and Signal Processing, pp. 27492752, Phoenix, AZ, (Mar. 1999). 
Chang et al., "A New Estimation Scheme For Frequency and Timing Offsets in OFDM Systems," Proc. of the IEEE Vehicular Technology Conference, pp. 18321835, Boston, MA, (Sep. 2000). 
Classen et al., "Frequency Synchronization Algorithms for OFDM Systems suitable for Communication over FrequencySelective Fading Channels," Proc. of The IEEE Vehicular Technology Conference, pp. 16551659, Stockholm, Sweden, (Jun. 1994). 
Gunther et al., "A New Approach for Symbol Frame Synchronization and Carrier Frequency Estimation in OFDM Communications," Proc. of The IEEE Int. Conf. on Acoustics, Speech and Signal Processing, pp. 27252728, Phoenix, AZ, (Mar. 1999). 
Hwang et al., "Frequency and Timing Period Offset Estimation Technique for OFDM Systems," Electronics Letters, vol. 34, No. 6, pp. 520521, (Mar. 19, 1998). 
Jin et al., "The Estimation of Time Delay and Doppler Stretch of Wideband Signals," IEEE Trans. on Signal Processing, vol. 43, No. 4, pp. 904916, (Apr. 1995). 
Kang et al., "Decisiondirected maximum likelihood estimation of OFDM frame synchronization offset," Electronics Letters, vol. 30, No. 25, pp. 21532154, (Dec. 8, 1994). 
Lambrette et al., "OFDM Burst Frequency Synchronization by Single Carrier Training Data," IEEE Communications Letters, vol. 1, No. 2, pp. 4648, (Mar. 1997). 
Lim et al., "An Efficient Carrier Frequency Offset Estimation Scheme for an OFDM System," Proc. of the IEEE Vehicular Technology Conference, pp. 24532457, Boston, MA, (Sep. 2000). 
Luise et al., "Carrier Frequency Acquisition and Tracking for OFDM Systems," IEEE Trans. on Communications, vol. 44 No. 11, pp. 15901598, (Nov. 1996). 
Moose et al., "A Technique for Orthogonal Frequency Division Multiplexing Frequency Offset Correction," IEEE Trans. on Communications, vol. 42 No. 10, pp. 29082914, (Oct. 1994). 
Pollet et al., "Synchronization with DMT Modulation," IEEE Communications Magazine, pp. 8086, (Apr. 1999). 
Rinne et al., "An Equalization Method for Orthogonal Frequency Division Multiplexing Systems in Channels with Multipath Propagation, Frequency Offset and Phase Noise," IEEE, pp. 14421446, (1996). 
Schmidl et al., "Blind synchronisaation for OFDM," Electronics Letters, vol. 33, No. 2, pp. 113114, (Jan. 16, 1997). 
Simoens et al., "A New Method for Joint Cancellation of Clock and Carrier Frequency Offsets in OFDM receivers over Frequency Selective Channels," 2000 IEEE 51<SUP>st </SUP>Vehicular Technology Conference Proceedings, pp. 390394, Tokyo, Japan, (May 2000). 
Tureli et al., "Blind Carrier Synchronization and Channel Identification for OFDM Communications," Proc. of IEEE Int. Conf. on Acoustics, Speech and Signal Processing, pp. 35093512, Seattle, WA, (May 1998). 
Visser et al., "A Novel Method for Blind Frequency Offset Correction in an OFDM System," Proc. of the IEEE PIMRC, pp. 816820, Boston, MA, (Sep. 1998). 
Wei et al., "Synchronization Requirement for Multiuser OFDM on Satellite Mobile and Twopath Rayleigh Fading Channels," IEEE Trans. on Communications, vol. 43, No. 2, pp. 887895, (Feb. 1995). 
Cited By (23)
Publication number  Priority date  Publication date  Assignee  Title 

US20040202234A1 (en) *  20030411  20041014  Agency For Science, Technology And Research  Lowcomplexity and fast frequency offset estimation for OFDM signals 
US7746760B2 (en)  20040108  20100629  Qualcomm Incorporated  Frequency error estimation and frame synchronization in an OFDM system 
US20050152326A1 (en) *  20040108  20050714  Rajiv Vijayan  Frequency error estimation and frame synchronization in an OFDM system 
US8619841B1 (en)  20040227  20131231  Marvell International Ltd.  Transceiver with carrier frequency offset based parameter adjustment 
US8311152B1 (en) *  20040227  20121113  Marvell International Ltd.  Adaptive OFDM receiver based on carrier frequency offset 
US20080205540A1 (en) *  20040528  20080828  Daisuke Takeda  Wireless communication apparatus 
US20060291549A1 (en) *  20050627  20061228  Nokia Corporation  Automatic receiver calibration with noise and fast fourier transform 
US7453934B2 (en) *  20050627  20081118  Nokia Corporation  Automatic receiver calibration with noise and fast fourier transform 
US7649964B2 (en) *  20060208  20100119  Nec Corporation  Radio receiver and noise estimated value correction method 
US20070183537A1 (en) *  20060208  20070809  Nec Corporation  Radio receiver and noise estimated value correction method 
US9088958B2 (en)  20070321  20150721  WiLan Inc.  Methods and apparatus for identifying subscriber station mobility 
US20080233945A1 (en) *  20070321  20080925  Nextwave Broadband Inc.  Methods and Apparatus for Identifying Subscriber Station Mobility 
WO2008115782A1 (en) *  20070321  20080925  Nextwave Broadband Inc.  Methods and apparatus for identifying subscriber station mobility 
US8064913B2 (en)  20070321  20111122  WiLan Inc.  Methods and apparatus for identifying subscriber station mobility 
US9775002B2 (en)  20070321  20170926  WiLan Inc.  Methods and apparatus for identifying subscriber station mobility 
US8457252B2 (en) *  20070404  20130604  Thomson Licensing  Method and apparatus for digital signal reception 
US20100150283A1 (en) *  20070404  20100617  Thomson Licensing A Corporation  Method and apparatus for digital signal reception 
US20110149724A1 (en) *  20091217  20110623  Electronics And Telecommunications Research Institute  Apparatus and method for transmitting/receiving data in wireless communication system 
US8599817B2 (en) *  20091217  20131203  Electronics And Telecommunications Research Institute  Apparatus and method for transmitting/receiving data in wireless communication system 
US8848844B2 (en) *  20100917  20140930  Telefonaktiebolaget L M Ericsson (Publ)  Receiver node and a method therein for compensating frequency offset 
US20130170590A1 (en) *  20100917  20130704  Telefonaktiebolaget L M Ericsson (Publ)  Receiver Node and A Method Therein for Compensating Frequency Offset 
CN102185811A (en) *  20110524  20110914  中国工程物理研究院电子工程研究所  Carrier frequency estimation method 
CN102185811B (en)  20110524  20140101  中国工程物理研究院电子工程研究所  Carrier frequency estimation method 
Also Published As
Publication number  Publication date  Type 

US20030128751A1 (en)  20030710  application 
Similar Documents
Publication  Publication Date  Title 

Morelli et al.  Frequency ambiguity resolution in OFDM systems  
US5912876A (en)  Method and apparatus for channel estimation  
US5627863A (en)  Frame synchronization in multicarrier transmission systems  
US5444697A (en)  Method and apparatus for frame synchronization in mobile OFDM data communication  
US7139321B2 (en)  Channel estimation for wireless OFDM systems  
US6628738B1 (en)  Method of arrangement to determine a clock timing error in a multicarrier transmission system, and a related synchronization units  
US6584164B1 (en)  Method for forming a training sequence  
US20100166050A1 (en)  Time error estimation for data symbols  
US7292527B2 (en)  Residual frequency error estimation in an OFDM receiver  
US7139320B1 (en)  Method and apparatus for multicarrier channel estimation and synchronization using pilot sequences  
US7457231B2 (en)  Staggered pilot transmission for channel estimation and time tracking  
US7532693B1 (en)  Method and apparatus for acquistion and tracking of orthogonal frequency division multiplexing symbol timing, carrier frequency offset and phase noise  
US20110026577A1 (en)  Equalization for OFDM Communication  
US20050190800A1 (en)  Method and apparatus for estimating noise power per subcarrier in a multicarrier system  
US20040001563A1 (en)  Robust OFDM carrier recovery methods and apparatus  
US20020145971A1 (en)  Apparatus and method for synchronizing frequency in orthogonal frequency division multiplexing communication system  
US7009932B2 (en)  Frequency tracking device for a receiver of a multicarrier communication system  
US20050163257A1 (en)  Channel estimation for a communication system using spectral estimation  
US7340000B1 (en)  Decision feedback equalizer in an OFDM WLAN receiver  
US20070070882A1 (en)  OFDM demodulating apparatus and method  
US20050084025A1 (en)  Timing offset compensation in orthogonal frequency division multiplexing systems  
US7039004B2 (en)  Decision feedback channel estimation and pilot tracking for OFDM systems  
US20050163263A1 (en)  Systems and methods for frequency acquisition in a wireless communication network  
US7184495B2 (en)  Efficient pilot tracking method for OFDM receivers  
US20070086328A1 (en)  Method and circuit for frequency offset estimation in frequency domain in the orthogonal frequency division multiplexing baseband receiver for IEEE 802.11A/G wireless LAN standard 
Legal Events
Date  Code  Title  Description 

AS  Assignment 
Owner name: RESONEXT COMMUNICATIONS, INC., CALIFORNIA Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:VANDENAMEELELEPLA, PATRICK;REEL/FRAME:012674/0169 Effective date: 20011213 

AS  Assignment 
Owner name: RF MICRO DEVICES, INC., NORTH CAROLINA Free format text: MERGER;ASSIGNOR:RESONEXT COMMUNICATIONS, INC.;REEL/FRAME:013957/0507 Effective date: 20021219 

FPAY  Fee payment 
Year of fee payment: 4 

AS  Assignment 
Owner name: CAMBRIDGE SILICON RADIO LIMITED, UNITED KINGDOM Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:RF MICRO DEVICES, INC.;REEL/FRAME:028520/0697 Effective date: 20120626 

FPAY  Fee payment 
Year of fee payment: 8 

AS  Assignment 
Owner name: QUALCOMM TECHNOLOGIES INTERNATIONAL, LTD., UNITED Free format text: CHANGE OF NAME;ASSIGNOR:CAMBRIDGE SILICON RADIO LIMITED;REEL/FRAME:036663/0211 Effective date: 20150813 